It's a cute coop and would be an attractive option for someone who doesn't want to build their own. These small coops always pose their own unique challenges. There are just a few changes and ideas I might suggest:
1. It needs more ventilation up at the ridge line of the roof when rising moisture and ammonia gas might accumulate.
2. It needs ventilation that doesn't rely solely on windows. When the weather turns wet and windy, I'd want to be able to close the windows and still have good ventilation without drafts.
3. It needs nest boxes. If they are inside then that reduces the number of chickens the coop can hold. You say it holds 3-6, but at 3'x4' then 3 full-sized hens is more realistic. Once you add one nest box (3 hens can share one) inside then the coop might hold 2-3 hens. Nest boxes also need to be somewhat private and out of the way.
4. It needs roosts that aren't over the nest boxes, feeders, waterers, etc.
5. Make it as easy to clean as possible. That might be a hinged bottom to dump the litter, poop trays, removable roosts, etc. Having to bend over and climb in that hatch to clean the interior will get old pretty quickly. Without poop trays under the roosts, the coop will need daily cleaning. If people instead rely on DLM, then the coop needs to be able to hold 6 - 8" of litter and a way to shovel or dump it out twice a year. How does that much litter affect the nest boxes and pop door?
6. The ramp to the pop door looks too steep, but that may just be the angle of the photo.
